Volume controlled sound reinforcement system



April 26, 1949. E. w. KELLOGG 2,468,205

VOLUME CONTROLLED SOUND REINFORCEMENT SYSTEM Filed Dec. 31', 1.946

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film/4R0 WKELLOGG wwwkm xsu SQ Patented Apr. 26, 1949 VOLUME CONTROLLEDSOUND REINFORCE- MENT SYSTEM Edward W. Kellogg, Haddonfield, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication December 31, 1946, Serial No. 719,452

6 Claims.

This invention relates to signal transmitting and receiving systems,such as public address and sound reinforcement systems, and particularlyto the automatic volume control of such systems.

General systems of this type are well-known in the art, reference beingmade to Anderson U. S. Patent No. 2,189,306 of February 6, 1940, and toStanko U. S. Patent No. 2,338,551 of January 4, 1944. The type of publicaddress system shown in the Anderson patent is one wherein the gain of avariable gain amplifier is at a stable value during no-signaltransmission and is increased by the signal during the transmissionthereof. If there is incipient singing while speech is beingtransmitted, it is promptly stopped when the speaker pauses betweensentences. This is accomplished by rectifying a portion of the signaland applying it to the grid of the variable gain amplifier.

In a large proportion of sound reinforcement systems, the microphone isacoustically coupled to the loud speakers, and if the gain oramplification exceeds a certain critical value, feedback or singingoccurs. tion on the usefulness of the system, and various expedientshave been proposed, to permit operation at higher gain without singing.The Anderson circuit prevents sound from the loud speakers fromsustaining high-gain condition by utilizing only the low frequencycomponents of the voice for the gain control, which frequencies arepresent in the voice, but not needed for intelligibility, and are nottransmitted to the loud speaker. Thus, the output of the speakercontains no frequencies which will be picked up by the microphone andtransmitted to the gain controlling rectifier.

It has been found, however, that in many instances, the first parts ofmany words are frequently lost in transmission because they consist ofunvoiced components, such as certain consonants, which have no stronglow-frequency components, and are, therefore, incapable of raising thegain. These initial sounds, which are very important to intelligibility,are thus reproduced at low amplification, and being usually of lowerintensity to start with than the remainder of the word, may fail to beheard.

The present invention, therefore, is directed to a circuit which willpermit the unvoiced components to increase the gain of the amplifier,but only as a transient condition during the time it takes for thevoiced components to act through the main gain control rectifier toincrease the This places a serious limitat the amplifier in the gain ofthe variable gain amplifier. The general problem of causing high as Wellas low frequency components of speech to exercise a desired influence ona variable gain amplifier has been recognized by Miller in his U. S.Patent No. 2,312,260 of February 23, 1943, wherein he utilizes anequalizer to vary the spectral energy distribution of the signal asapplied to the rectifier; but, the Miller patent does not deal with theproblem of stability.

The present invention accomplishes the result of an increase in gain bythe unvoiced components by utilizing an additional rectifier on which isimpressed the higher frequencies of the signal, but applies the voltagederived by rectification of the high frequency components in such a waythat it is effective toward increasing gain only during the interimrequired for the low frequency components to act on the gain control.

The principal object of the invention, therefore, is to facilitate thetransmission of signals in a system in which feedback can occur.

Another object of the invention is to provide an improved method ofcontrolling the gain of a variable gain amplifier with the signalcurrents in a system Where the input and output are acousticallycoupled.

A further object of the invention is to provide an improved non-feedbackpublic address system, wherein the variable gain amplifier is controlledby both the voiced and unvoiced components of words.

A still further object of the invention is to provide an improvedcontrol circuit for a Variable gain amplifier.

Although the novel features which are believed to be characteristic ofthis invention will be pointed out with particularity in the appendedclaims, the manner of its organization and the mode of its operationwill be better understood by referring to the following description readin conjunction with the accompanying drawing, in which the single figureis a schematic circuit diagram of a public address system embodying theinvention.

Referring to the diagram, a microphone 5 is connected over conductors 6to a preamplifier I having dual output circuits. The outputs of theamplifier 1 are passed into two filters, filter I0 including two seriescondensers H and I2 and a shunt inductance l3, and passes frequenciesabove 500 cycles only, while filter i5 is composed of two seriesinductances l6 and I! and a shunting condenser l8 and passes frequencies below 500 cycles only. Although a 500 cycle cut-ofi point isgiven as an illustration, it is to be understood that the cut-off pointmay be varied, while it may also be desired to use a band-pass filter atl which passes frequencies between 100 and 150 to 500 cycles. In the useof such a band-pass filter, any background noise having frequenciesbelow 100 or 150 cycles will not be transmitted to vary the gain of thevariable gain amplifier.

The filter I0 is coupled by a transformer to an amplifier tube 2|,and'a'lso to an amplifier tube 22. The signal is amplified in amplifiertube 22 and then impressed on a variable gain amplifier 24, composed ofpush-pull. tubes 25 and 26, over a transformer 21. The output of thevariable gain amplifier is coupled over transformer '29 to an amplifierfeeding a loud speaker 3|.

The amplifier 30 may also include a compressor, if desired. Thus, thesignal is transmitted from the microphone 5 to a loud speaker 3| overthe filter Ill and amplifiers 22, 24, and 30, this signal beingcomposed, however, only of frequencies above the cut-off of the filterIn, as for example those above 500 cycles.

The low frequency portion of the signal passed by filter I5 is impressedon an amplifier 33 over transformer 32 and then over conductor 34 andtransformer 35 to a full-wave rectifier 31 composed of tubes 38 and 39.The output of the "rectifier is connected toa ripple removing filtercomposed of resistor 40, condensers 4| and a resistor 49, and then overconductor 42 and biasing battery 44 to the gain control electrodes ofthe tubes 25 and 26 of the variable gain amplifier 24. The portion ofthe circuit just described is comparable to the circuit shown in theaboveidentified Anderson Patent No. 2,189,306, although a limitingdevice, composed of -a rectifier tube 41 and biasing battery 48, isprovided between the ripple filter and the variable gain amplifier toprevent the voltage on the grids of the variable gain amplifier fromgoing beyond a predetermined operating value.

As mentioned above, however, the circuit so far 7 described does notprovide the necessary increase in gain. whenever the first word startswith an unvoiced consonant. To permit the gain to be increased by suchunvoiced consonants, the remainder'of the circuit shown has beenprovided. This portion of the circuit consists of the amplifier 2|connected to the filter H) in which the high frequency components of-thesignal are amplified and which are then impressedover con- :ductor 52and transformer 53 on a full-wave rectifier 54 consisting of tubes 55and'56-.- The output of the rectifier 54 is transmitted over a rippleremoving filter composed of a. resistor 60, condensers 6|, and dischargeresistor 62, to the conductor 42 over a condenser 64. The circuitcomposed of amplifier 2| and rectifier 54 thus receives the signal astransmitted to speaker 31, and will contain the signal frequencies above500 cycles. The voltage produced by these signal frequencies will beimpressed through condenser 64 on the grids of tubes 25 and 26. However,since the direct current output of the rectifier 54 is impressed on thegrids through condenser 64, the positive voltage impressed on thecontrol grids of amplifiers 25 and 26 can last only for a brief period,for as soon as condenser 64' is charged, no further current can flow tosustain the voltage across discharge resistor 49. Thus, the-continuationof input torectifier 54 will not maintain amplifier 24 in the high'gain'condition. However, the gain change is produced over a sum-- oientperiod to give the low frequency control circuit, composed of amplifier33 and rectifier 31, time to increase the gain of the variable gainamplifier with the low frequency components of the signal. The time thatis required for the voltage increase caused by the high-frequencycomponents to leak away, is determined by the capacities of condensersB4 and 4| in relation to resistor 49.

The above circuit, therefore, provides the nonfeedback condition asdisclosed in the abovementioned Anderson patent, and also provides gainchanges for all portions of the signal, thereby preventing the loss ofcertain portions of the signal not having the required energy content tooperate the normal gain control rectifier.

I claim:

1. In combination, a variable gain amplifier, a signal source connectedto the input of said amplifier, a loud speaker connected to the outputof said amplifier, filter means connected between said amplifier andsaid signal source for selectin'g predetermined frequencies from saidsignal source for impression on said amplifier, a rectifier connected tosaid filter means for producing a voltage in accordance with saidpredetermined frequencies in the output of said microphone, means forconnecting the output of said rectifier to said variable gain amplifierfor controlling the gain thereof in accordance with said predeterminedfrequencies, a second filter means connected to said signal source, asecond rectifier connected between said second filter means and said andsaid variable gain amplifier for limiting the gain control of saidvariable gain amplifier by said predetermined frequencies to a certaininitial time period.

2. In a signal transmission circuit, a microphone for detecting signals,a variable gain amplifier' connected to said microphone, a loud speakerconnected to said variable gain amplifier, a rectifier, means connectedbetween said microphone and said variable gain amplifier, for selectingpredetermined frequencies in the output of said microphone forimpression on said variable gain amplifier and on said rectifierforvarying the gain of said variable gain amplifier, means connectedbetween said rectifier and said variable gain amplifier to limit thevarying of said variable gain amplifier by said predeterminedfrequencies to a predetermined time period, and means for selectingother frequencies in the output of said microphone for continuouslyvarying the gain of said variable gain amplifier.

3. A signal transmission circuit in accordance with claim 2, in whichsaid first mentioned means includes a low pass filter, and said lastmentioned means includes a high pass filter and a second rectifier.

4. A signal transmission circuit comprising a variable gain amplifier,an input circuit for said amplifier selective to a predetermined rangeofsignal frequencies,a-=rectifier circuit connected to said inputcircuitandselective to a different range of signal frequencies, saidrectifier circuit being connected to said variable gain amplifier forcontinuously varying the gain thereof; a rec tifier circuitconnected tosaid input circuit and adaptedtovary the gain of said variable gainamplifier, said second mentioned rectifier circuit being sensitive tothe same range of frequencies as said input circuit, and an electricalelement connected between said second mentioned rectifier circuit andsaid variable gain amplifier for varying the gain thereof for a shortinitial time period.

5. A signal transmission circuit in accordance with claim 4, in whichsaid input circuit and said second mentioned rectifier circuit areselective to a high frequency range and said first mentioned rectifiercircuit is selective to a low frequency range.

6. A signal transmission circuit in accordance with claim 4, in whichsaid input circuit and said second mentioned rectifier circuit areselective to an audio frequency range above substantially 500 cycles.

EDWARD W. KELLOGG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

